Assessment of Catchment-Scale Evapotranspiration in a Process-Based Integrated Model Via Boundary Condition Switching Vs Root Water Uptake Modeling

Abstract:

Evapotranspiration (ET) is one of the fundamental terms of the hydrologic cycle at all scales, yet it is also one of the more difficult to model, being influenced by many factors, such as air temperature and humidity, soil moisture, vegetation species, and horizontal advection. Therefore, practical applications of hydrological process-based models of coupled surface and subsurface flow where ET plays a significant role are typically subjected to large uncertainties. A simplified method for computing ET, implemented in the integrated model CATchment HYdrology (CATHY) and based on a switching procedure for the boundary conditions (BCs) at the soil surface, has been recently demonstrated capable of reproducing measured ET in a semi-arid catchment in southeastern Australia, mostly covered by shallow-rooted vegetation. Here we run the model in the same Australian catchment to compare the BC-switching method with the commonly used root water uptake model by Feddes. We investigate i) the trade-offs between the two ET calculation methods, ii) the maximum root depth at which the BC-switching method is able to match catchment-scale ET without losing physical meaning, and iii) the impact of oxygen stress (not included in the BC-switching method) on the main terms of the catchment water balance. A comprehensive sensitivity analysis on the parameters of the Feddes' model pointed out that the maximum root depth is the major control on catchment-scale evapotranspiration and streamflow. The subsequent comparison with the BC-switching method highlighted that the latter is suitable to reproduce ET when the root depth of the vegetation does not exceed approximately 50 cm. For deeper rooting systems, the BC-switching not only fails to match the fluxes, but is also affected by numerical artifacts making the simulated drying and wetting processes physically meaningless. These findings are confirmed by further simulations run with a hypothetical wetter scenarios. Finally, we show that oxygen stress can have a significant impact on both ET and streamflow, even though these effects do not depend much on the soil moisture value that defines oxygen stress onset.